The idea of implementing all the optical functions of a laser time of flight velocimeter (LTV) in a single diffractive optical element (DOE) was proposed in EP 0 770 219. The system of EP 0 770 219 was used for velocity measurements on solid surfaces. A major advantage of the system is that the calibration of the system is inherent in the diffractive optical element and is to first approximation independent of the wavelength, and therefore makes it possible to use inexpensive and non-stabilised laser diodes.
The LTV system is schematically illustrated in FIG. 1. A collimated beam, originating from a laser diode with a collimation lens, is incident on the central part of the DOE. The DOE diffracts, splits and focuses the beam into two foci, that make up the measurement volume of the sensor. Particles, carried by the flow, that passes the foci will scatter light in all directions. Some light will be scattered in the backward direction and is collected by the receiver part of the DOE. This is a lens that diffracts and images the measurement volume onto the two detectors. Particles that passes the measurement volume will give rise to a signal peak in each detector, with a time delay. When this time delay and the distance between the foci is known then the particle velocity can be determined, and hence the flow velocity.
The diffraction angle of DOEs is highly dependent on the wavelength, and even for temperature stabilised laser diodes the emitted wavelength will be undefined within 1-3 longitudinal modes. This is due to modehops and hysteresis in the temperature dependence. For a laser at 785 nm this implies that the wavelength uncertainty is approx. ±0.3 nm. This is illustrated in FIG. 2, where the measured emitted peak wavelength is shown as a function of the temperature of the laser diode.
For a LTV flow sensor it is important to have a spatially well-confined measurement volume in which the focused laser beams are parallel to a certain degree of accuracy. This is partly achieved by applying narrow apertures in front of the detectors (confocal system). The receiver diffractive lens has to image the measurement volume in the flow onto the detector apertures. However, if the diffraction angle of the receiver part differs from the angle of incidence of the transmitter part, the image position in the detector plane will depend on the wavelength. When having large detectors this is not a problem, but when narrow apertures are inserted, in order to reduce cross-talk and limit the measurement volume, this is a severe problem.
It is an object of the present invention to provide a solution of the above-mentioned problem—i.e. to provide a sensor configuration wherein the image position in the detector plane is independent of wavelength.